In most existing mobile communication systems, for example, various wireless communication systems such as Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access Wireless (WCDMA), Code Division Multiple Addressing (CDMA), Worldwide Interoperability for Microwave Access (WiMAX), CDMA2000, TD-SCDMA, Long Term Evolution (LTE) and WLAN/WiFi, a random access procedure has to be performed first when a terminal initiates an uplink service. At the point, access collisions may occur and cause the access failed due to the possibility that a plurality of terminals may initiate respective random access procedures concurrently with the same access occasion and the same access code selected. Access failure may cause increasing access delay and a lower successful access rate. In current mobile communication systems, a conventional way to ensure the access delay and the success rate is to provide more access resources so as to lower the collision probability.
With the development of mobile communication techniques, machine to machine (M2M) communication has been proposed. M2M and IOT (Internet of Things) become a hotspot that operators concern, and various applications have been developed rapidly based on M2M and IOT. Different from prior H2H (human to human) communication, M2M communication requires an even larger number of terminals, perhaps up to 10 times of that for H2H or even more, for example, a magnitude order of 50 billion.
IOT refers to applications of M2M in WANs, especially in mobile operators' networks, that is, wireless data transmission services that take the mobile network operators' wireless networks as a platform and provide industry machine-to-machine services in various transmission manners (such as, CDMA, GSM, etc) through special industry terminals.
M2M/IOT has extensive fields of applications, and is applicable to intelligent transportation, telemedicine, monitoring, smart grids, environmental monitoring, intelligent home and other situations.
MTC (Machine Type communication) communication refers to network communication that takes place in one network element (NE) or between a plurality of NEs without human interventions (i.e., an M2M application), such as, traffic control and management, factory monitoring, remote meter reading and the like. In M2M applications, a plurality of M2ME (Machine to Machine Equipment) having the same M2M application form an integral unit also called as a group.
MTC device identity can uniquely identify a M2M terminal, which may employ IMSI (International Mobile Subscriber Identity), MSISDN (Mobile Station International ISDN Number), IP address, IMPU (IP Multimedia Public Identity), IMPI (IP Multimedia Private Identity) and the like.
Due to limited wireless communication resources, time-frequency resources are allocated only when a user wants to communicate. When a user initiates a communication, first of all, a random access procedure has to be performed, followed by connection establishment and data transmission according to RRC (Radio resource control), and finally, allocated resources are released after the communication. Taking LTE as an example, random access resources comprise two parts: slot and frequency for initiating a random access; and preamble to be used. In existing cellular wireless communication systems, random access mainly takes two forms: contention based and non-contention based random accesses.
Non-contention based random access refers to notifying UE (User Equipment) by eNB (Enhanced NodeB) to use its particular access resources. Because access resources have to be allocated by eNB, it may result in a larger system overhead. So, it is only used in situations with higher delay requirements, such as handover and dropped connection.
In the contention based random access, eNB may broadcast information about access resources by which random access may be initiated in a cell, and then UE may randomly select an allowed preamble, and randomly select a random access frequency resource at a next time that allows random access, and then initiate a random access at the selected time-frequency position. Because each UE randomly selects random access resources in an independent manner, when the same random access resources are selected by a plurality of UEs, conflicts may occur. According to a solution for resolving the conflict in the existing protocols, only one of the plurality of UEs in conflict may have successful access, and other UEs need to initiate their access attempts again after a back-off period, and withdraw if conflicts occur again, until access succeeds or a maximum attempt number is reached. The occurrence of conflict may delay UE access and put an extra burden on the network and UEs.
The number of terminals engaged in M2M communications is very huge, while each terminal may have a very small interaction traffic amount and the interaction may be a burst interaction. A M2M terminal (also called as MTC terminal) may realize a relatively simple function and only need to communicate some particular information; some M2M terminals may have relatively constant positions, or may be seldom moved; M2M terminals may have consistent behaviors in a special application, such as intelligent meter reading.
In M2M services, because there are a large number of MTC terminals, network resource contention occurs if the large number of MTC terminals access the network, putting a further burden on the network. It is worse especially in some applications such as intelligent meter reading.
Taking intelligent transportation as an example, a typical intelligent transportation system comprises a GPS (Globe Positioning System)/GLONASS (Global Navigation Satellite System of Russia) satellite positioning system, a vehicle mounted mobile terminal, a wireless network, and a ITS (Intelligent Transport System) control center. The vehicle mounted terminal receives ranging information from a navigation satellite network through the GPS module and transmits longitude, latitude, speed, time and other information of the vehicle to a microcontroller; vehicle status information is collected by a video equipment; the microcontroller communicates interactive information in two-way with the ITS control center through a GPRS module so as to realize vehicle monitoring and other functions.
As another example about a smart grid, the smart grid requires all intelligent grid terminals to periodically report their data, for example, in the electronic meter reading service, all intelligent grid terminals upload their data to a particular server at a certain time. For example, in many countries, smart grids instruct a large number of MTC terminals to frequently report their meter reading data, which has only a small amount of data, with a period of 5 minutes.
In existing networks, for example, such as LET, the random access procedure is performed for individual UEs. A contention based random access procedure can be described as follows.
A UE randomly and equiprobably selects access resources within a range that is permitted, and sends a selected preamble at the selected resource position.
101. The UE monitors a random access response (RAR) returned from a eNB, where the RAR comprises random access resource information that has been received by the eNB, timing advance (TA) information, UL resource grant (UL GRANT) for a next stage, and temporary cell radio network temporary identity (T-CRNTI).
102. If the UE detects a RAR for its own selected random access resources, step 103 is executed; otherwise, a preamble is re-initiated.
103. The UE performs UL synchronization according to TA, and sends a request message, including the unique identity of the UE, at a resource position indicated by the UL GRANT, and it can also performs HARQ (Hybrid Automatic repeat reQuest).
104. When the eNB correctly receives the request message sent from the UE at 103, it returns an correct receiving message with the unique identity of the UE; then, only the UE that receives the correct receiving message correctly and has a matched identity returns an ACK, while other UEs withdraw and attempt their access later, by which the problem of conflict is solved.
However, when too many users initiate access attempts simultaneously, multiple UEs may select the same random access resource, causing significantly increasing of average access delay for users. Meanwhile, a large amount of re-initiated access attempts may cause large overheads on UEs and eNB.
Therefore, when a large number of terminals transmit their data, it may cause a significant increase in average access delay for users and a large signaling overhead, which may affect normal communications.